Reciprocating Refrigeration Compressor and Method for Mounting a Reciprocating Refrigeration Compressor

ABSTRACT

The compressor includes: a crankcase carrying a cylinder and a bearing hub which lodges a crankshaft; a valve plate closing one end of the cylinder; a piston reciprocating in the cylinder and driven by the rotation of the crankshaft; an electric motor having a stator affixed to the crankcase and provided with winding grooves and teeth, each tooth carrying a respective shoe, and a rotor affixed to the crankshaft and carrying magnet segments, stator, the rotor and the crankshaft are mounted in an indexed way, in order to present, in relation to one another, a relative positioning which produces, upon the stop of the electric motor and with the crankshaft and rotor being in the upper dead point condition of the piston, a cogging torque capable of taking the piston away from the upper dead point.

FIELD OF THE INVENTION

The present invention refers to a compressor of the type provided with a piston which is driven, in a reciprocating linear movement in the interior of a cylinder, by a connecting rod coupled to the eccentric of a crankshaft affixed to the rotor of an electric motor, the electro-mechanical assembly of the compressor, including the crankshaft, the electric motor, the connecting rod and the piston, being constructed and mounted in order to avoid the stop of the piston in the upper dead point or vicinities thereof, when the electric motor turns off to stop the compressor.

BACKGROUND OF THE INVENTION

In the refrigeration compressors having a reciprocating piston, the compression chamber is defined in the interior of the cylinder, between the top of the piston and a valve plate, containing the suction and discharge valves and which is affixed to the crankcase of the compressor, closing one of the ends of the cylinder.

In the type or compressor considered herein, the stop of the piston in the upper dead point, or its vicinities, reduces de sealing capacity of the discharge valve(s), significantly increasing the return of the refrigerant fluid, from the condenser of the refrigeration system to the compressor region maintained under low pressure.

The chart represented in FIG. 1 of the appended drawings shows the pressure reduction in the discharge region of the compressor, as a function of the position of the eccentric of the crankshaft, after the motor turns off to stop the compressor. The position of 180° represents the upper dead point of the piston, whereas the positions 0° and 360° represent the lower dead point.

It may be noted that the pressure drop rate, which represents the leak through the discharge valve(s), is at least five times greater in the situations in which the compressor stops with the piston in the upper dead point) (180° or vicinities thereof.

The reduction of the sealing capacity of the discharge valve(s), due to the stop of the piston in the upper dead point or vicinities thereof, is explained by the pressure conditions reigning in the discharge region of the compressor, as schematically represented in FIG. 2 of the appended drawings. In the situation in which the piston 20 stops in the upper dead point or close to it, the pressure difference between the interior of the compression chamber C and the downstream side of the discharge valve(s) 40 a, in the interior of the cover 41 of the cylinder 11, is small, reducing the force resulting from the required pressure differential with the function of closing the discharge valve(s) 40 a, pressing it (them) against the respective discharge seat defined in the valve plate 40. If there is not a minimum pressure differential between the two sides of the discharge valve(s) 40 a, the force for closing the discharge valve(s) 40 a will not be enough to guarantee a more efficient closing for impeding the passage of gas through the respective seat in the valve plate 40.

After leaking through the not completely closed discharge valve(s) (40 a), the refrigerant fluid flows through the radial gap existing between the cylinder 11 and the piston 20 until the region of the compressor and refrigeration system, which are under low pressure.

Excessive leaks through the discharge valve(s) 40 a after the stop of the compressor are undesirable since, upon flowing from the condenser to the evaporator of the refrigeration system, passing through the compressor, the refrigerant fluid contained in the low pressure side of the refrigeration system will absorb the thermal energy contained in the body of the compressor, transferring said thermal energy to the interior of the refrigeration equipment in which the evaporator of the system is installed. Such process increases the thermal load in the evaporator, impairing the energetic efficiency of the refrigeration system.

Moreover, the leaks through the discharge valve(s) 40 a, after the stop of the compressor, reduce the energy saving obtained by using check valves installed between the condenser and the expanding device and whose function is to avoid a new compression of the refrigerant fluid already stored, at high pressure, in the condenser of the refrigeration system.

Due to the losses caused to the energetic efficiency of a refrigeration system, upon the stop of the compressor with the piston 20 in the upper dead point, it is desirable to provide a technical solution which can avoid such operational condition.

SUMMARY OF THE INVENTION

Considering the facts discussed above and related to the state of the art, the present invention has the object to provide a reciprocating refrigeration compressor, which is constructed in order to avoid, upon the stop of the electric motor of the compressor, the piston from remaining in the upper dead point or in positions close to the latter, in which the pressure differential between the downstream side and the upstream side of the discharge valve(s) is not sufficient to guarantee the tight closing of said valve(s).

It is a further object of the invention to provide a new method for mounting the type of reciprocating compressor considered herein.

These and other objectives of the invention are achieved by a reciprocating refrigeration compressor of the type which comprises: a crankcase carrying a cylinder and a bearing hub which lodges a crankshaft provided with an eccentric; a valve plate closing one end of the cylinder; a piston reciprocating in the interior of the cylinder and driven by a connecting rod coupled to the eccentric of the crankshaft; an electric motor having a stator affixed to the crankcase and provided with a plurality of winding grooves intercalated with teeth, each tooth carrying a shoe adjacent to the air gap of the stator, and a rotor affixed to the crankshaft and carrying magnet segments, which are circumferential and peripheral.

According to one aspect of the invention, the stator, the rotor and the crankshaft present, in relation to one another, a relative positioning which produces, upon stop of the electric motor and with the assembly defined by the crankshaft and rotor being in the piston upper dead point position, a cogging torque capable of taking the piston away from the upper dead point, guaranteeing the sealing of the discharge valves. The cogging torque is defined as the torque resulting from the variation of the reluctance to the passage of the useful magnetic flow, generated by the magnet, between at least one magnet segment of the rotor and a confronting stator tooth.

The invention further provides a method for mounting the compressor cited above, according to which the rotor is affixed to the crankshaft, which is already housed in the bearing hub and which is retained with its eccentric in a position corresponding to the piston upper dead point position and in which a cogging torque is generated capable of taking the piston away from the upper dead point.

The present proposed construction does not require any relevant constructive alteration to be made in the compressor. However, according to the constructive solution proposed herein, if a stop of the electric motor of the compressor occurs in a position of the rotor-crankshaft assembly in the piston upper dead point condition, at least one of the magnet segments of the rotor will produce, with a confronting stator tooth, a cogging torque capable of taking the piston away from the upper dead point and its vicinities, after the stop of the motor, creating an unstable balance position, allowing the magnetic flows circulating through the air gap to force the rotor and the crankshaft to jointly rotate to a position of minimum reluctance to the passage of the magnetic flow, stabilizing the rotor-crankshaft assembly in a position in which the piston is displaced away from its upper dead point, by a distance capable of producing a pressure differential between the interior of the compression chamber of the cylinder and the downstream side of the discharge valve(s), sufficient to guarantee the closing of the discharge valve(s).

The solution proposed by the invention transforms the usual stop system of the electric motor of the compressor, in which the piston has aleatory positions in the interior of the cylinder, in a controlled system through which, upon occurring a stop of the motor with the piston in the upper dead point position or close to said position, the rotor-crankshaft assembly will be magnetically turned to the stable balance position, taking the piston away from the upper dead point.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, by making reference to the appended drawings, by way of example and in which:

FIG. 1 represents a chart illustrating the pressure reduction in the discharge region of the compressor, as a function of the piston stop angle upon the stop of the electric motor of the compressor;

FIG. 2 illustrates, schematically, the excessive gas leak condition through the discharge valve upon the stop of the electric motor of the compressor with the piston in the upper dead point position;

FIG. 3 represents a schematic plan view of the assembly formed by the stator, rotor, crankshaft, connecting-rod and piston in the upper dead point position, in which the rotor has a pair of magnet segments defining, with the respective teeth and shoes of the stator, a condition in which the cogging torque is capable of taking the piston away from the upper dead point and vicinities thereof;

FIG. 4 represents a view similar to that of FIG. 3, but illustrating the piston away from its upper dead point position, with the crankshaft being additionally turned clockwise from the upper dead point position of the piston, with the magnet segments of the rotor defining, with the respective teeth and shoes of the stator, a stable condition, that is, of minimum reluctance to the passage of the useful magnetic flow formed between said magnet segments and the respective teeth and shoes of the stator; and

FIG. 5 represents a view similar to that of FIG. 4, but illustrating the piston away from its upper dead point position with the crankshaft being additionally turned anti-clockwise from the upper dead point position of the piston, with the magnet segments of the rotor defining, with the respective teeth and shoes of the stator, a stable condition, that is, of minimum reluctance to the passage of the useful magnetic flow formed between said magnet segments and the respective teeth and shoes of the stator.

DETAILED DESCRIPTION OF THE INVENTION

According to the illustrations and as already mentioned, the present invention is applied to a refrigeration compressor, more specifically a reciprocating compressor, hermetic or not, of the type described above and which presents, in the interior of a shell (not illustrated), a crankcase B, usually formed in cast iron and presenting a usually flat outer face F.

The crankcase B carries at least one piston hub 10 which defines a cylinder 11, having one end open to the outer face F of the crankcase and in whose interior is housed and displaced, in a reciprocating linear movement, a piston 20. The cylinder 11 has said end closed by a valve plate 40 and by a cylinder cover 41, which are seated and affixed, usually by screws (not illustrated), against the outer face F of the crankcase B, as illustrated in FIG. 2. The valve plate 40 carries at least one discharge valve 40 a and one suction valve, not illustrated, said valves being of the blade type and presenting any construction adequate to the operation by pressure differential between its upstream and downstream sides, in order to establish a selective fluid communication between a compression chamber C, defined in the interior of the cylinder 11 between the piston 20 and the valve plate 40 and the suction and discharge sides of the compressor.

The crankcase B further carries a bearing hub 50 which houses a crankshaft 60 provided with an eccentric 61 in which is coupled one end of a connecting rod 62 whose opposite end is coupled to the piston 20, in order to displace the latter in a reciprocating movement in the interior of the cylinder 11, between upper dead point and lower dead point positions, by applying rotation to the crankshaft 40.

The compressor is driven by an electric motor M having a stator 70 affixed to the crankcase B and provided with a plurality of winding grooves 71 intercalated with teeth 72 which carry, each one, respective shoe 72 a, and a rotor 80, affixed to the crankshaft 60 and carrying magnet segments 81, which are disposed substantially in a peripheral circumferential alignment.

According to the invention, one of the ways for mounting the electro-mechanical assembly of the compressor is accomplished by affixing the stator 70, in a determined designed position, in the crankcase B of the compressor. After the position of the stator 70 is defined in relation to the crankcase B and, consequently, to the cylinder 11 which is defined in the piston hub 10, the crankshaft 60 is mounted in the bearing hub 50 of the crankcase B and rotatively positioned in the upper dead point of the piston 20.

Once the crankshaft 60 is mounted and angularly positioned in the bearing hub 50, the rotor 80 of the electric motor M may be affixed to a corresponding extension of the crankshaft 60, in a relative angular positioning, which allows, when the eccentric 61 of the crankshaft 60 is in the upper dead point position of the piston 20, generating a cogging torque capable of taking the piston away from the upper dead point and from vicinities thereof.

More specifically, the proposed solution provides the fixation of the rotor 80 to the crankshaft 60 in a relative position in which the symmetry radial axis of at least one magnet segment 81 coincides with the symmetry radial axis of a corresponding winding groove 71, with said symmetry radial axes being positioned between the two teeth 72 adjacent to said winding groove 71.

According to the invention, the indexation for fixation of the crankshaft 60 in the rotor 80 and the fixation of the stator 70 in the crankcase B, with the indexation of at least one stator groove 71, in relation to the rotational position of the assembly of crankshaft 60 and rotor 80, corresponding to the upper dead point position of the piston 20, allows that, in this position of the piston 20, the magnetic forces (not illustrated), which are present in the rotor-stator assembly, are used to prevent the piston from remaining, in a stationary condition, in the upper dead point position, if said position of the piston occurs upon a stop of the electric motor of the compressor.

In the upper dead point position of the piston 20, the magnetic forces acting over the rotor 80 create a condition of great instability for the latter, forcing its additional rotation, in one or in the other sense, by an angle which corresponds to 1/p the angular distance between each two consecutive positions of instability condition of the rotor 80, in which p is the number of poles of the rotor.

Considering that the rotor 80 has an even number of magnet segments 81, the angular distance between each two consecutive positions of magnetic instability of the rotor 80 will correspond to the ratio of 360° for half the product of the number of magnet segments 81 by the number of stator grooves 71. In the example illustrated in FIGS. 3, 4 and 5, the rotor has four magnet segments 81 (four poles) and the stator 70 has six stator grooves 71. In this case, the motor M presents 12 magnetic instability positions (unstable nature) and 12 magnetic stability positions (stable nature) of the rotor 80, which positions are intercalated between one in relation to the other, each two consecutive positions, of the same nature, being angularly spaced from each other by an angle of 30° and also spaced from the adjacent or consecutive positions of other nature, by an angle of 15°.

In the present solution, whenever the rotor 80 stops in a condition of unstable magnetic balance in relation to a diametrically opposite pair of magnet segments 81, it will be magnetically forced to rotate in one or in the other sense, towards a stable magnetic balance condition generated by the magnetic field (not illustrated), by an angle which will depend on the number of poles of the rotor 80 and of the stator grooves 71. In the illustrated example, the angle will be of at least 7.5 degrees in any of the senses. In a rotor of six poles, operating with a stator of nine grooves, said angle of additional magnetic displacement will be of at least 5 degrees. 

1. A reciprocating refrigeration compressor, comprising: a crankcase carrying a cylinder and a bearing hub which lodges a crankshaft provided with an eccentric; a valve plate closing one end of the cylinder; a piston reciprocating in the interior of the cylinder and driven by a connecting rod coupled to the eccentric of the crankshaft; an electric motor having a stator affixed to the crankcase and provided with a plurality of winding grooves intercalated with teeth, each tooth carrying a respective shoe, and a rotor affixed to the crankshaft and carrying magnet segments; the reciprocating refrigeration compressor characterized in that, in the condition of upper dead point of the piston, at least one magnet segment of the rotor has its symmetry radial axis coinciding with the symmetry radial axis of a corresponding winding groove, said symmetry radial axes being positioned between the two stator teeth adjacent to said winding groove.
 2. The compressor, according to claim 1, characterized in that the stator, the rotor and the crankshaft present, in relation to one another, a relative positioning which produces, upon the stop of the electric motor and with the crankshaft and rotor assembly being in the upper dead point condition of the piston, a cogging torque capable of taking the piston away from the upper dead point.
 3. A method for mounting the reciprocating refrigeration compressor according to claim 1, the method being characterized in that it comprises the steps of: affixing the stator to the crankcase; mounting the crankshaft in the bearing hub of the crankcase, rotatively locking the crankshaft with its eccentric in the upper dead point position of the piston; affixing the rotor in the crankshaft in a relative angular positioning, in order to produce a cogging torque capable of taking the piston away from the upper dead point position.
 4. The method, according to claim 3, characterized in that the rotor is affixed to the crankshaft, with the symmetry radial axis of at least one magnet segment coinciding with the symmetry radial axis of a corresponding winding groove, said symmetry radial axes being positioned between the two teeth adjacent to said winding groove.
 5. The method, according to claim 3, characterized in that the rotor is affixed to the crankshaft with the diametral direction of the crankshaft, passing through the geometric center of the eccentric, coinciding with the symmetry radial axes of a magnet segment and of the corresponding winding groove. 